1. Field of the Invention
The present invention relates to a steer-by-wire system wherein a steering handle of a vehicle is mechanically disconnected from a pair of steerable wheels and wherein a steering motor steers the steerable wheels in dependence on the manipulation of the steering handle while a reaction force motor generates a steering reaction force against the manipulation of the steering handle.
2. Discussion of the Related Art
In conventional vehicles, when a steering handle is turned to steer steerable wheels from a neutral position (the position in which the steerable wheels are to be for traveling the vehicle straight ahead) during the traveling, a road reaction force (the road reaction force in this case is called a “self-aligning torque) acts for returning the steerable wheels to the neutral position. In a steering system (hereafter referred to as “steering system for conventional vehicles”) having a steering handle and steerable wheels connected mechanically, the road reaction force is conveyed to the steering handle through a mechanical transmission train, and the driver feels the road reaction force as a steering reaction force against the manipulation of the steering handle.
Also in steer-by-wire systems, it has been desired that the driver can feel a steering reaction force against the manipulation of a steering handle. To this end, in prior steer-by-wire systems, the steering reaction force is falsely generated based on information such as vehicle steering position, vehicle speed and the like as described in, e.g., Japanese unexamined, published patent application No. 2002-145099 (Patent Document 1), whose system will be referred to as “prior steer-by-wire system 1”. Alternatively, as those described in Japanese unexamined, published patent applications No. 2003-81111 (Patent Document 2) and No. 2004-34923 (Patent Document 3), a reaction force which steerable wheels of a vehicle receive from the road surface is detected, and a steering reaction force is generated in dependence on the detected road reaction force, the system of which will be referred to as “prior steer-by-wire system 2”.
By the way, steer-by-wire systems generally take a construction that a ball screw mechanism or a rack-and-pinion mechanism is provided between an inter-steerable wheel shaft, interposed between a pair of steerable wheels, and a steering motor, and the mechanism is inherently accompanied by mechanical friction. This friction acts as a resistance against the returning of the steering handle to the neutral position in the steering system for the conventional vehicles and makes a cause to deteriorate the returning of the steering handle. The same phenomenon occurs in the prior steer-by-wire system 2 as exemplified in the Patent Documents 2 and 3 and gives rise to the problem.
In the prior steer-by-wire system 1 as exemplified in the Patent Document 1, on the other hand, the phenomenon which is caused by such friction to deteriorate the returning of the steering handle does not arise, but another problem arises in that the state of the road surface is not conveyed to the driver because a steering reaction force which directly reflects the road reaction force cannot be obtained as given in the steering system for the conventional vehicles.
As a prior art example for solving the problem involved in the prior steer-by-wire system 1, there is known a method which as described in the Patent Document 2, adds the steering reaction force which reflects the disturbance of the road surface as given in the prior steer-by-wire system 2, to the steering reaction force given in the prior steer-by-wire system 1. However, because the steering reaction forces given in the prior steer-by-wire systems 1 and 2 are only added simply, the prior art example cannot make use of the advantages of the both systems and hence, is inferior in steering feeling to the steering system for the conventional vehicles.